In many communication networks using optical fibers that transmit digital information by amplitude modulation of a usually infrared light beam, so-called multimode optical fibers are used. Unlike single-mode fibers that have a core with a very small diameter and that spread light in a single mode, which is the fundamental mode, multimode fibers have a core with a larger diameter and can propagate light simultaneously in several propagation modes. The propagation modes energized in the fiber are characterized by electric field phase and intensity spatial profiles in a plane transverse to the propagation axis; such profiles are different, depending on the modes, and several modes can co-exist. Multimode fibers are advantageous in that they can transmit more power than a single-mode fiber when the beam applied to the inlet has several modes; a single-mode fiber would just eliminate the energy supplied in the modes other than the fundamental mode. Moreover, connecting multimode fibers with each other or with other components, including sources and receivers, is easier, because of the larger diameter of their cores: side positioning and angular alignment tolerances are more flexible. These fibers are compatible with those of laser radiation sources that are themselves multimode ones, like the Vertical Cavity Surface-Emitting semiconductor laser diodes (VCSEL), which are the sources easiest to industrially manufacture.
But multimode fibers have a drawback, which is modal dispersion: several modes can simultaneously propagate in the fiber, but the propagation speed of light varies according to the propagation mode. Such discrepancy in the propagation speed is very small but it plays a non-negligible role in long fibers. As a matter of fact, it results, at a remote end of the fiber, in a possible mixing of the digital information, which modulates the intensity of a light beam injected at the other end. A narrow light pulse injected on the source side becomes a spread pulse on the receiver side. For a long fiber, it may happen that one information bit propagated by a slow mode reaches the end of the fiber at the same time as the next bit propagated by a fast mode. Decoding the digital information in the receiver may become difficult, if the pulses are transmitted at high speed and/or if the fiber is long. This results in a bandwidth limited by the modal dispersion; such modal bandwidth limits the maximum data rate that can be reached in the fiber, depending on the length thereof.
This problem has been partly solved by a more sophisticated design of the fibers, so that standard multimode fibers (OM1, OM2, OM3, OM4) exist, which have progressively improved the balancing of the various propagation speeds by optimizing the index profiles of the core of the fiber (step-index or graded-index profiles, or more complex profiles). As an order of magnitude, the limit rate of one 300-meter long fiber OM3 is of the order of 10 Gbit/second, but is reduced to 1 Gbit/second for a 600-meter long fiber and to 100 Mbit/second for a 2,000-meter long fiber.
Improving such speeds in already deployed communication networks, for instance, corporate networks or data centers, or in new networks, is desired. The solution consisting in changing the fibers to replace the same with multimode fibers having a higher standard, or even with single-mode fibers, is an expensive solution, both with regard to hardware (higher standard fibers are more expensive) and set-up time, in existing networks.
One solution already proposed to try to solve such problem consists in installing mode filters to eliminate either the faster modes or the slower modes, while keeping the modes that have speeds in a limited range only. The main drawback is the cost of such filters and, more specifically, the resulting energy loss, since the eliminated modes are, by nature, modes that otherwise transport a part of the radiation energy. Such energy loss, which is added to natural losses in the long fibers, makes the detection of information in the receiver, at the end of the fiber, more difficult.
Other solutions may have been proposed that use improved electronic processing for demodulating the transmitted information, and that specifically use adaptive filters. Solutions using several successive fibers having different properties or several parallel fibers having different properties, or even several cores of fibers having different properties in a single fiber, have also been proposed.